This invention relates generally to the art of heat-treatment of dielectric materials by means of a radio frequency (RF) electromagnetic field, and more specifically to the selection or modification of materials to form the mold to provide for uniform curing of thermoplastics and related materials by radio frequency heating. The concepts of the present invention are adapted to the molding of three-dimensional objects of different shapes.
In curing processes utilized for forming thermoplastics and the like into molded articles of varying shape, the uncured or unprocessed plastic material is placed in a mold and then heat is applied to the plastic material to cure the material, causing the material to set in the desired shape and strengthening the plastic material through cross-linking of plastic molecules. One method for heating the plastic material in the mold is through radio frequency or RF heating in which the mold is positioned between two electrodes across which an electromagnetic field may be applied to heat the working material or plastic positioned in the mold. In such a process, the electromagnetic field is applied across the electrodes during a heating cycle or period to raise the temperature of the working material to a target temperature or selected temperature to achieve the desired curing effect. RF heating can also be used for curing wood objects which are placed in a mold like structure between electrodes across which an RF electromagnetic field is applied to heat the wood product and any water contained therein to drive off the water.
The use of RF heating in molding applications and the like has given rise to many problems not only in the selection of suitable mold materials for particular plastics or other working materials, but also with the overall concept of the process as well. In molding processes, it is desirable that curing be uniform across the entire volume of the plastic pieces formed. For this purpose there are two conditions which must be satisfied: (1) the power dissipated in the plastic has to be uniform throughout the volume of the plastic article, which provides uniform temperature distribution throughout the plastic article's volume at any moment during the heating cycle; and (2) the heat flow on the interface of the mold and the plastic has to be as small as practical during the entire heating cycle, which means that the gradient of the temperature at their interface has to be equal to zero, or be as close to zero as practical during the heating cycle.
Prior concepts for uniform curing of dielectric material have been developed. One considers the relation between the loss factors of mold and work materials as a control factor of uniform curing. Loss factor is defined as the product of a dielectric constant .di-elect cons., and power factor tan .delta.. For example, U.S. Pat. No. 2,754,546 teaches the minimization of heat flow from the article to be molded substantially by equalizing the loss factor of the inner mold layer adjacent the plastic material defining a mold cavity. U.S. Pat. No. 2,626,428 teaches the use of an isolated inner insert in a mold containing a filler with power losses considerably higher than that of the mold material to compensate for heat flow from the plastic to the mold.
Variations of these concepts may be found in several other patents. For example, U.S. Pat. No. 2,438,952 selects a dielectric material of relatively lower power factor than the tire being cured, providing some selectivity in the heating of the tire. U.S. Pat. No. 2,407,833 obtains uniform heating of a fibrous structure of different thickness by using an adhesive of high dielectric loss factor at the thicker cross-sections of an article, and an adhesive of relatively lower loss factor at thinner cross-sections of the article. All of the above described patents, analyzing heat exchange on mold-article interface, take into the account only the loss factor or power factor, but no other physical parameters are considered. A significant drawback of the methodology of all of the above mentioned patents is that their concepts, based on the loss factors of the mold and the work materials, are correct only in limited cases when mechanical and thermal properties of the mold and the work materials are very close. In cases when the mold and work material have different properties, this concept will not work. As it will be shown hereafter, in general cases, to maintain zero heat flow on the mold-plastic interface during a radio frequency heating process, physical parameters such as the power factor divided by the product of the specific heat and density tan .delta./c.rho. should be substantially the same for corresponding mold and work materials. The prior patents fail to consider these necessary additional factors. Moreover, loss factors of mold and work materials may considerably differ from each other. For example, in the case of silicon rubber as a mold material and plastic foam as a work material with equal parameters of thermosensitivity (tan .delta./c.rho.), their loss factors tan .delta. will be very different due to the difference of their density c and specific heat .rho..
Another concept of uniform curing of work material discussed in the prior art is based on the matching of dielectric constant .di-elect cons. of the mold and work materials. Examples of such can be found in U.S. Pat. No. 2,421,097 and U.S. Pat. No. 4,441,876. As it is shown hereafter, the matching of the dielectric constants .di-elect cons. of the mold and work materials is a necessary condition to provide uniform dissipation of RF energy throughout the work volume. However, uniform dissipation of RF energy is not equivalent to a uniform temperature profile or field across the work material due to heat flow on the mold-plastic interface. Therefore, another condition, approximate equality of the thermosensitivity of the material to the RF field, is necessary to maintain zero heat flow on the mold-plastic interface. The above mentioned prior art patents ignore the heat exchange between the work material and the mold. As will be shown below, this factor must be considered as well to maintain zero heat flow on the mold-plastic interface.
The failure of the prior art patents to discuss the heat exchange between the work material and the mold explains certain disadvantages of the methods discussed therein. As an illustration, let us consider the definition of power factor, which is the basic definition of the theory of dielectric heating. In many publications the introducing of the power factor tan .delta. applies to parallel-plate capacitors and neglects the fringing effect. However, RF heating is available not only with flat electrodes, but with concentric cylindrical or spherical electrodes as well. In such cases, the concept of parallel-plate capacitors of unit size will not be applicable because the force lines extend radially outward and they are not parallel to the side surface of the parallel-plate capacitor.
Another disadvantage of the applied theory of some prior art patents is that they consider only exact matching of the dielectric constants of the mold and work materials, and do not investigate the influence of mismatching of the dielectric constants, .di-elect cons., to the uniformity of the dissipated RF energy. It is known that dielectric constant of some thermoplastics may vary considerably with changes in temperature. Therefore, if in the beginning of the heating cycle there is proper matching of dielectric constants, the dielectric constant of the work or plastic material will change its value during heating, causing mismatching with dielectric constant of the mold material. This will result in a distortion of the field, which will differ for areas of different thickness and thus, by the end of the heating cycle or period the temperature distribution inside the work material will be uneven.